| 摘要 |
1402530 Intruder alarms MICROWAVE & ELECTRONIC SYSTEMS Ltd 8 Dec 1972 [10 Dec 1971] 57598/71 Heading G4N Apparatus for detecting damage to sheet material, e.g. glass, capable of propagating wave energy at ultrasonic frequencies comprises: a transducer 11 attached to a sheet of material 14 to provide electrical signals corresponding to vibrations propagated in the sheet; a filter 32 responsive to the electrical signals and stopping frequencies less than 100 kHz; and means responsive to the filtered signal to provide an output. Signals from a number of piezoelectric transducers at locations 10a...10c are combined at 22 and, after passing through high-pass filter 32, are applied to two amplifiers 33, 53. Frequencies corresponding to the cutting of glass are detected at 34 and shaped into rectangular pulses by Schmitt trigger 36. The output of the trigger 36 is fed to an integrator 38, e.g. a diode transistor pump which, after a predetermined number of pulses have been received, produces an output of sufficient level to operate threshold circuit 42 and energize alarm relay 44. Higher level signals produced by breaking of the glass are fed through the channel comprising amplifier 53, detector 54 and Schmitt trigger 56, the amplitude of the output from trigger 56 being large enough for the integrator 38 output to operate threshold 42 immediately instead of after a delay. The link between the transducers and the unit 22 may be radio or cable. If cables are used, further circuitry 50, 51 may be provided to initiate an alarm in the event of tampering with the cables. In a modification, Fig. 3 (not shown), the trigger 36 is omitted, and the same channel is used for the cutting and shattering signals since the signals at the integrator input are then amplitude dependent. |
